1. Field of the Invention
The present invention describes a process for preparing, purifying and isolating .beta.-hydroxyalkylamides which are used as chemical intermediates and as chemical crosslinkers for carboxyl-functional polyesters and acrylates in solvent-based surface coatings and powder coatings. The use in powder coatings in particular places very high demands on the physical form of the .beta.-hydroxyalkylamides. Only free-flowing powders, i.e. not soft, sticky or waxy powders, are suitable for use in powder coatings.
2. Description of the Prior Art
.beta.-Hydroxyalkylamides are prepared by aminolysis of alkyl esters by .beta.-aminoalcohols in the presence of basic catalysts such as sodium hydroxide or sodium methoxide, with the .beta.-aminoalcohols being used in excess in most cases wing to the selectivity of the reaction. In the case of liquid .beta.-hydroxyalkylamides, the unreacted .beta.-aminoalcohols have to be removed from the reaction mixture before the .beta.-hydroxyalkylamides can be used. In the case of solid .beta.-hydroxyalkylamides, the isolation and purification of the .beta.-hydroxyalkylamides is carried out either by crystallization in a solvent (J. Coat. Tech., 50(643), 49-55 (1978), U.S. Pat. No. 4,076,917, U.S. Pat. No. 4,727,111) or, specifically in the case of solid .beta.-hydroxyethylamides, directly from the reaction mixture in a solvent-free slurry process (U.S. Pat. No. 5,101,073).
In the case of crystallization in solvents, the .beta.-hydroxyalkylamides are generally either added to a hot solvent such as methanol and/or acetone or the solvent is added to the .beta.-hydroxyalkylamides. After cooling the solution and crystallization, the .beta.-hydroxyalkylamides are then filtered off and freed of solvent by drying. The yield is reduced by the solubility in the solvent used. In addition, the catalyst remaining in the reaction mixture can lead to undesirable secondary reactions, e.g. to diacetone alcohol when using acetone as solvent, which also results in losses in the recovery of the solvent used. It is also found that unreacted .beta.-aminoalcohols also coprecipitate as undesired impurities in the crystallization and, in addition, .beta.-aminoalcohols act as solubilizers which have an adverse effect on the crystallization. As a result, the yield of .beta.-hydroxyalkylamides is reduced further.
Specifically the preparation of solid .beta.-hydroxyethyl-amides can be carried out in the melt in a solvent-free slurry process. The slurry process (U.S. Pat. No. 5,101,073) is based on the equilibrium reactions which proceed in the preparation of .beta.-hydroxyethylamides being shifted in the direction of the desired end product .beta.-hydroxyethylamide, the desired .beta.-hydroxyethylamide being precipitated from the melt by heating in a particular temperature range and the melt crystallizing as a result. In the case of substances where the desired .beta.-hydroxyalkylamide does not precipitate from the melt, e.g. in the case of .beta.-hydroxypropylamides, the slurry process fails. In addition, the slurry process is restricted to the use of equimolar amounts of alkyl esters and .beta.-hydroxyethylamines. The slurry process gives, when equimolar amounts of dialkyl esters and .beta.-hydroxy-ethylamines are used in the presence of basic catalysts such as sodium hydroxide or sodium methoxide, not only the desired monomeric .beta.-hydroxyethylamides (I) but also, as by-products, dimers (II) and esteramides (III). Furthermore, the reaction product still contains .beta.-hydroxyethlamine. ##STR1## ##STR2##
where m=0 to 10 and R is a C1-C5-alkyl group.
An important process step in the preparation of .beta.-hydroxyalkylamides by aminolysis of alkyl esters by .beta.-aminoalcohols in the presence of basic catalysts such as sodium hydroxide or sodium methoxide is the removal of unreacted .beta.-aminoalcohols from the reaction mixture.
The separation of the excess .beta.-aminoalcohol by dissolution in a suitable solvent such as methanol and subsequent removal of the .beta.-aminoalcohol by means of an ion exchanger is known from the abovementioned literature. The desired .beta.-hydroxyalkylamide is subsequently obtained by distilling off the solvent. However, this method is only suitable for the laboratory scale, since the removal of by-products by means of ion exchangers is only suitable for separating off small amounts of by-products. If a high proportion of by-products is present, i.e. when using a large excess of .beta.-aminoalcohol in the reaction, such a process is very complicated and uneconomical in industry.